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Apparently he doesn't actually know the equation (although his question didn't make that apparent) but he would like to know the gradient. He was hoping Excel might be advanced enough to be able to calculate it.

This is what I did in Excel 2002 (aka Excel XP). I'm not sure if it is the same in Excel 2000 or Excel 97.

I created a set of data that I knew would create a linear graph with a slope of 2 and a vertical axis intercept of 3, and then used the chart wizard to create that graph.

Right-click on the line (the graph).
From the context-sensitive menu, selct "Add Trendline..."
When the Add Trendline box appears, select the Options tab.
Check the box that says,"Display equation on chart".
Click OK.
The equation of the trendline will now be displayed. Since the trendline for a linear graph is the very same line, the equation of the trendline is (conveniently) the equation of the line itself.

The equation of the trendline is displayed in the form y=mx+c, and m=the slope, or gradient.